If rapid temperature changes are encountered, transient analysis may be required due to thermal stresses. One bearing should be fixed, serving as an anchor bearing to locate the shaft lengthwise.
A shaft cooler is a small fan that clamps on the shaft between the fan housing and the inboard bearing. There a few ways to insulate a fan, but the main idea is to create a second fan housing offset from the main fan housing and then fill the gap between the two with insulation.chicagoblowercanada. During maintenance repairs, it is very important that the overhang dimension is never increased. should be used for higher temperatures up to 1800F. When the temperature exceeds 800F, it is necessary to separate the fan pedestal from the fan housing.com/propellerfans.When fans and blowers are exposed to elevated temperatures, many aspects of the fan design must be reviewed to make sure the fan will withstand the extreme temperatures and that the fan is appropriately designed for the application. Temperature derates lower the maximum speed of a wheel to account for the lower material strength at elevated temperatures. The shaft cooler draws cool air over the shaft and bearings as the shaft rotates to dissipate the heat. Generally, the pedestal must be modified to allow room for the shaft cooler.ca
CBC Canada Co. The wheel must sustain the highest stress of all the fan components. By doing this, the amount of heat conducted through the pedestal and to the bearings and motor is greatly reduced. The shaft and bearings may require a cooling system or other means to keep them from overheating. As the overhang dimension is increased, the shaft safe speed becomes lower. The bearings should have proper lubrication that can handle the higher operating temperatures.
The space created between the inboard bearing and the fan housing allows for the use of a shaft cooler and shaft seal. Bearing temperatures should be monitored and should not be allowed to exceed 200F at speeds higher than 2500 RPM and 220F at speeds below 2500 RPM.html
The bearing closest to the drive end is normally fixed. For higher temperature fans, water cooling can be used to cool the shaft. Standard axial fans are good to 200F. Again, the material chosen also depends on the stress levels and the material properties at elevated temperatures. For example, the maximum temperature for a standard fan that does not have a bearing in the airstream can run up to 300F. All other bearings should be expansion bearings to permit the shaft to move lengthwise. Heat resistant alloys such as stainless steels (304, 316, 347), Incalloy 800HT, 230, Inconel (600, 625, 617), Multimet, Haynes 25, etc.
Up to this point, the discussion has centered on centrifugal fans. The wheel material should be selected to provide adequate strength at the elevated temperature. A customer may want to keep the heat of the airstream from heating up the fan housing and radiating out to the surrounding area.
This dimension, referred to as the overhang dimension, is critical in determining the safe speed of the shaft. High temperature fans, if designed properly, can provide years of dependable service. Sometimes, it is necessary to consider high cycle fatigue, low cycle fatigue in cyclical operation with speed changes, and numerous start-stop cycles.
A shaft cooler is recommended for all applications over 300F. Bearings should be selected to allow for free movement of the shaft lengthwise due to temperature changes.
Bearings usually require high temperature lubricants and sometimes circulating oil or static oil in a monoblock. The inboard bearing, which is the bearing nearest the fan housing, should be moved away from the fan housing to create some space for the heat to hoodedfans. This includes, but is not limited to, the wheel, shaft, bearings, and housing. For higher temperatures, stainless steels and Inconel may be required. Insulating the fan accomplishes this. This will increase the distance between the inboard bearing and the wheel.